1. Field of the Invention
This invention relates to integrated circuit devices, and more particularly to a method and apparatus for calibrating voltage-controlled devices in control loop circuits.
2. Description of Related Art
Voltage-controlled devices used as basic building blocks in a variety of practical applications. For example, transmitters typically include voltage-controlled oscillators (VCO) that operate over a wide frequency range. These VCOs include voltage-controlled circuit elements such as variable capacitors (i.e., varactors) that have wide tuning ranges.
Disadvantageously, voltage-controlled circuit elements having wide tuning ranges are extremely sensitive to small changes in the control voltage (i.e., small changes in the control voltage result in large changes in output frequency), which is undesirable in practical circuit applications such as VCOs.
One method of implementing a VCO that attempts to overcome the above described sensitivity problem uses digitally controlled “coarse tune” circuit elements (e.g. switched capacitors) in addition to “fine tune” circuit elements (e.g. a varactor having a narrow tuning range) to provide operation over a wide range of frequencies. One such method is described in the above-incorporated U.S. Pat. No. 6,323,736 (hereinafter referred to as the '736 patent). One of the VCO implementations described in the '736 patent is shown in FIG. 8. As shown, FIG. 8 includes a plurality of digitally controlled capacitors 802 and a pair of varactors 806 and 806′ arranged in a parallel configuration. The varactors provide fine tune capabilities (i.e., small changes in output frequency) in response to a control voltage VTUNE 810; the plurality of digitally controlled capacitors provides coarse tune capabilities (i.e. large changes in output frequency for a given control voltage change). The plurality of digitally controlled capacitors, which may be controlled by digital control voltages Vc1, Vc2, Vc3 and Vc4, effectively increases the tuning range of the VCO. The '736 patent also describes a VCO as used in a phase lock loop (PLL) implementation.
One drawback of the coarse/fine tuning method is that the varactors used to implement the method are non-linear by nature, and thus tuning sensitivity (i.e., the ability to change capacitance or capacitance-related characteristics in accordance with a control voltage) varies greatly depending upon the control voltage. Thus, an application that implements the coarse/fine tune method may attempt to operate in a low sensitivity tuning range, which can limit its overall operating frequency range. Absolute varactor capacitance values depend on manufacturing process variations. Calibration is essentially a closed-loop procedure for allowing circuits to adapt to these variations.
Another drawback associated with the coarse/fine tune method is the lack of a means for calibrating control voltage drift due to temperature variations. As is well known, performance characteristics of circuits and devices can vary depending on temperature. For example, circuit temperature can increase during circuit operation from morning to afternoon; and temperature-dependent circuit characteristics can cause the control voltage to drift into tuning regions having low sensitivity. Thus, the frequency range of the PLL may be limited by the temperature variations. Accordingly, the effective operating temperature range of the circuit is limited without calibration.
Therefore, a need exists for a method and apparatus that overcomes these above-identified drawbacks. The need exists for an apparatus such as a voltage-controlled device in a control loop that maintains high tuning sensitivity. In addition, the need exists for an apparatus and method that can dynamically calibrate a voltage-controlled device such as a VCO for temperature changes. The present invention provides such a method and apparatus.